The present invention relates to a communication system comprising a constellation of a plurality of artificial satellites orbiting the Earth, and particularly to a communications system for use with mobile stations and land vehicles.
Fixed (as distinguished from mobile) ground stations communicating with satellites usually have their antennas in specially selected locations where an undisturbed line-of-sight to the satellite exists. In contrast, mobile stations (e.g., in land vehicles) have to be able to communicate from almost anywhere and so could be severely affected by shadowing from mountains and buildings and by propagation impairments due to foliage. These effects are severe at the low elevation look-angles to the satellite, especially in the northern latitude countries, looking at a satellite in geostationary orbit. The low elevation angles also cause multipath fadings and distortions making communication difficult; they may also cause interference with terrestial communications.
Satellite-based systems for communication with mobile stations, such as ships, aircraft or vehicles, are well-known, in use, or being developed. Examples are the INMARSAT-MARISAT network, the planned M-SAT in the USA, and location and data-communication systems like GEOSTAR, OMNITRACS, and ESA-PRODATA. All these systems use equatorial geosynchronous satellites (i.e., having 24-hour orbits) which are almost fixed in the sky relative to an Earth station. However, equatorial orbits produce low elevation angles at the higher latitudes where most of the highly developed areas of Europe, North America and Japan are located, thus making them disadvantageous for use with mobile ground stations which require high elevation angles as indicated above. High elevation angles also make it possible to use simpler upward-pointing antennas on the mobile ground stations, and simpler satellite antennas, and also reduces interference with communication links on land.
Non-equatorial orbits are also known. A doubly geosynchronous (12-hour) inclined elliptical orbit was first used by the USSR-MOLNIYA satellites to obtain coverage at all latitudes of the country. The constellation required three satellites, each being used for eight hours in one out-of-two orbits, the second orbit passing over the Pacific Ocean without being useful. Proposals for using similar inclined elliptical 12-hour orbits are being studied mainly to improve communications at higher latitudes and also to relieve congestion of the geosynchronous orbit; examples of such latter constellations include the German LOOPUS, the British T-SAT proposals and the ESA-ARCHIMEDES concept. Another elliptical inclined orbit is the ESA-proposed TUNDRA, which is a 24-hour orbit at higher apogee altitudes, similar also to the French SYCOMORES proposal.
Elliptical orbits are usually inclined about 63.4.degree. to the Equator for minimizing the fuel requirements to avoid perigee drift. For best coverage of the northern hemisphere, the perigee is kept at the most southern point of the orbit, and drifts in longitude can be adjusted by changes in the orbit period. However, constellations of three (or four) satellites would provide continuous communication only in the European area at an elevation angle of about 60.degree., which elevation angle would drop to about 45.degree. if the Middle East and North Africa are included. In order to cover also North America and the Far East, nine to twelve satellites would be required.
Recently, MOTOROLA proposed the IRIDIUM global mobile communication system using a constellation of 77 satellites in low earth orbits at about 765 Km altitude. However, such a constellation could assure continuous world-wide mobile communication with an elevation angle of only 20.degree. in the middle latitudes, and even down to only about 10.degree. globally. It appears that for higher elevation angles, an impractically large number of satellites would be required. Other proposals were also recently made using low Earth orbits and similar large constellations with low elevational angles.